There is a formal classification system for hypothyroidism.

Cause
There are many potential causes of low thyroid hormone production including :

• A failure of the thyroid gland to develop at all (called agenesis) ...results in congenital hypothyroidism
• Underdevelopment of  thyroid tissue mass (hypoplasia) is a cause of primary hypothyroidism (rare)
• Iodine deficiency during foetal development (rare) - results in congenital hypothyroidism, or in adulthood causes acquired hypothyroidism (rare)
• Autoimmune disease in which the body produces antibodies against the body's own thyroid tissue. These antibodies destroy the normal thyroid tissue replacing it with fibrous tissue. This is called lymphocytic thyroiditis and is one of the most common cause of primary hypothyroidism in dogs. In Beagles it may be a familial disorder...but it does not lead to hypothyroidism in all individuals.
• Degeneration of the thyroid tissue due to cell death (necrosis) or reduced tissue mass (atrophy) is also a common cause of hypothyroidism
• Cancer of the thyroid may cause primary hypothyroidism (rare)
• Lack of thyroid stimulating hormone (TSH) from the pituitary gland in the brain results in lack of stimulation of the thyroid to secrete it's hormones, resulting in secondary hypothyroidism - though this is uncommon (accounts for less than 5% of cases)
  • This can be due to a congenital defect in the pituitary and is associated with dwarfism because there is usually a lack of growth hormone as well 
  • This can be acquired as in cancer of the pituitary gland which results in reduced TSH secretion.
• Lack of secretion of thyrotropin releasing hormone (TRH) from the hypothalamus in the brain will result in a lack of production of TSH in the pituitary and so a lack of production of thyroid hormones - but this tertiary form of hypothyroidism has not been reported to occur in dogs
• Hypothyroidism can result following the surgical removal or chemical destruction or suppression of the thyroid gland as a treatment for thyroid cancer, and excess thyroid hormone secretion.. This is called iatrogenic hypothyroidism.

Signs
Clinical signs of hypothyroidism in an affected individual can vary because the thyroid hormones affect many organ systems. Some of the most common signs include :

• Lethargy
• Weakness and poor exercise tolerance
• Mental dullness
• Weight gain
• Hair loss (symmetrical on both sides of the body) on the flanks, neck, thighs and tail. Not usually itchy.
• Occasionally, in some breeds eg Irish Setters,  the coat becomes thicker
• Dull, dry, scurfy coat (dandruff). Occasionally itchy.
• Black pigmentation and thickening of the skin.
• Abnormal oestrus in bitches (irregular, prolonged bleeding), infertility, abortions and stillbirths, fading puppy syndrome (early post-natal deaths).
• Abnormal lactation and breast development
• Reduced size (atrophy) of the testicles in males, poor libido and poor sperm production
• Low body temperature
• Neurological disorders including facial paralysis, head tilt, laryngeal paralysis, loss of proprioception (knuckling of the feet), megaoesophagus, muscle wastage, stiffness, weakness
• Anaemia, bleeding disorders,a low white cell count (hence a poor immune response to infections)
• Slow heart rate and abnormal cardiac rhythm. Cardiomyopathy.
• Constipation, and sometimes vomiting or diarrhoea
• Aggression has been reported to be associated with atypical hypothyroidism 
• Provet is interested in feedback from you on some of these, and other signs of hypothyroidism CLICK HERE   

Provet is conducting an on-going  survey into the relationship between hypothyroidism and these and other clinical conditions  CLICK HERE for more information

In humans enlargement of the thyroid gland (seen as a swelling in the neck below the Adam's apple - called goitre) is a common feature of hypothyroidism but this is rare in dogs, except in association with thyroid cancer.

Complications
Severe hypothyroidism can lead to myxoedema (generalised body oedema) and coma. (very rare).

Diagnosis is based upon the clinical signs and laboratory tests, in particular the measurement of blood thyroxine (T4) concentrations will confirm the presence of hypothyroidism.:

• Basal - Normal range is 17-46 nmol/l in dogs. Unfortunately, the basal concentration of T4 can be affected by other disease situations - called the " euthyroid sick syndrome " (CLICK HERE for a list ) and so it is an unreliable measurement by itself. In addition several drugs can reduce the T4 concentrations CLICK HERE for further details.
• Thyroid Stimulating Hormone (TSH) stimulation test. Collect starved blood sample. Administer 0.1 IU TSH/ kg body weight IV. After 6 hours take second sample. Measure T4 in both samples. Hypothyroid dogs show little or no increase. Dogs with "euthyroid sick syndrome", or with drug interference show an increase in T4 in response to the TSH.

Other non-specific abnormal laboratory findings which may be present in hypothyroid patients include :

• Anaemia (normocytic, normochromic, non-regenerative)
• High blood cholesterol
• High blood triglyceride
• High blood creatine kinase
• Increased ALT, AST, ALT and LD
• Proteinuria - if auto-immune damage to kidney as well as thyroid

Stabilising hypothyroid patients can be tricky, especially if other disorders are present at the same time. 

Side effects are uncommon.

Because thyroid supplementation increases metabolic rate and activity in many tissues patients should be checked regularly (every 6 months) to ensure there are no signs of overdosage.

Long term problems
Atherosclerosis is rare in dogs, but it has been reported to occur in dogs with untreated hypothyroidism, and this disease can create serious circulatory difficulties for the patient.

Updated October 2013